1. Field of the Invention
The present invention relates generally to lime slurries and, more specifically, to a calcium hydroxide slurry produced with an organic liquid as a base.
2. Description of the Prior Art
Lime has a variety of uses. It is commonly used in treating waste water and sewage. It is used in agriculture to neutralize acidic soils and to provide nutrients for sustaining plant life. Lime is also used extensively in construction for the stabilization of soils and as a component in a variety of building materials. Lime is also used in a variety of “food grade” products intended for human consumption.
In this description, the term “lime” will be understood to mean both quicklime (calcium oxide) and hydrated lime (calcium hydroxide). Quicklime is produced by heating limestone (calcium carbonate) in a kiln at extreme temperatures to “calcine” the material and thereby drive off carbon dioxide. Quicklime is usually in the form of lumps or pebbles. In order to further process lime and improve the ease with which it is handled, dry lime is often mixed with water to form a slurry. In the case of quicklime, the water reacts with the quicklime in an exothermic reaction to form hydrated lime. This is often referred to as slaking. During the slaking of quicklime, large amounts of heat are given off which can significantly raise the temperature of the slurry. Water can then be driven off to produce dry, hydrated lime which is usually a powder. The term “lime” in this discussion will also be understood to encompass dolomite and dolomitic quicklime, i.e., CaO.MgOand Ca(OH)2.Mg(OH)2.
Worldwide, about 120 million tons of quicklime are produced every year (not including captive lime production and small lime producers in developing countries). As mentioned above, significant markets for lime products exist in such diverse industries as iron- and steel-making, chemicals generally, construction and civil engineering, gas and water treatment and the agriculture and food industries. In many of these commercial applications for lime products, quicklime is “slaked” as described above to form calcium hydroxide, Ca(OH)2, and is then finally used in the form of a suspension of hydrated lime particles in water, usually referred to as a “lime slurry”, or more accurately as a calcium hydroxide slurry.
The transformation of quicklime into hydrated lime may be accomplished by the customer, as by purchasing quicklime and slaking it “on site” or by purchasing hydrated lime and mixing it with water on site. Alternatively, the transformation may be completed directly at the producing lime plant (also by slaking quicklime in water or by mixing hydrated lime with water). In the latter case, the lime producers are able to apply additional process steps which allow the manufacture of specialty products with defined chemical and physical properties to match the specific needs of individual customers. Examples of such additional process steps include air classification of dry hydrated lime prior to mixing with water and the wet milling of calcium hydroxide slurries.
Such tailor-made and “ready to use” calcium hydroxide slurries become increasingly important in many applications, because they require less handling efforts and allow more accurate dosing as well as increased process automation. The viscosity and the sedimentation rate of such slurries are important quality parameters for these products, because they directly affect the flow properties and storage stability of the slurries.
The flow properties of these calcium hydroxide slurries mainly depend on solids content and particle size distribution and may vary from a product with the handling characteristics of a real liquid (sometimes referred to as a “low solids” slurry) to a semi-fluid paste (sometimes referred to as a “lime-putty”). A high solids content is important for the acid neutralization potential and the transport economy of these slurries because less water has to be transported. Additionally, an increased fineness of the hydrated lime particles in these tailor made slurries increases the chemical reactivity and reduces the sedimentation rate of these products.
Unfortunately, a higher solids content and an increased particle fineness are properties which both increase the slurry viscosity. Viscosities which are too high negatively affect the handling characteristics of the slurry. As a result, these two parameters usually have to be selected to maximize either the chemical reactivity or the solids content. In other words, a compromise is reached between the two parameters in order to maintain the desired properties of a liquid suspension.
U.S. Pat. No. 5,616,283, entitled “High Solids Lime As A Caustic Replacement”, issued Apr. 1, 1997, and assigned to the assignee of the present invention is an example of the production of a calcium hydroxide slurry with a very high solids content (between 35 and 55%) which also maintains desirable chemical reactivity and flow properties.
An example of the production of a low solids calcium hydroxide slurry which is designed with a very fine particle size to maximize the chemical reactivity is given in German patent DE 27 14858 C3, issued Oct. 2, 1986 to Schaefer. In the production method described in the Schaefer patent, optimized slaking conditions and high shear rates applied during slaking are stated to be primarily responsible for the well dispersed and finely divided Ca(OH)2 particles which are obtained in that process. Mean particle sizes below 1 μm are stated to be possible.
Another technique which can be utilized to obtain calcium hydroxide slurries with very fine particle sizes and increased chemical reactivity and handling characteristics is the wet milling of “conventional lime slurry” in a bead mill. The conventional lime slurry can be made by either slaking quicklime in water, or by mixing hydrated lime, Ca(OH)2 with water. The properties of the final calcium hydroxide slurry depend upon the particle size of the starting hydrated lime, the throughput rate of the mill and the type and size of the grinding media used in the bead mill. Mean particle sizes of 2 μm and below are possible by utilizing these techniques.
As a result of the high particle-fineness, the solids contents of such ultra-fine calcium hydroxide slurries are usually limited (≦20 wt-%) to maintain acceptable flow properties and handling characteristics.
In almost all the end applications in which such optimized, fine, stable calcium hydroxide slurries are used, the water added in the production process by the calcium hydroxide slurry does not cause any adverse effects. One example would be the use of ultrafine calcium hydroxide slurry for the softening of drinking water. However, there are some applications, such as the addition of hydrated lime to bitumen to achieve a lime-modified asphalt-binder or the use of hydrated lime in the production of organic solvent based oil, grease and lubrication products, in which water introduced by the calcium hydroxide slurry causes problems or has the potential to cause problems.
In order to minimize the amount of water present, these production processes either utilize a calcium hydroxide slurry with a high solids content (which still contains more than 50 wt-% of water) or utilize a hydrated lime which is added as a dry powder. In these cases, it is usually not possible to optimize the properties of the hydrated lime, especially in terms of chemical reactivity, degree of dispersion or sedimentation behavior.
Additionally, when “normal” commercial hydrated lime is added to organic solvents, the Ca(OH)2 particles have a tendency to agglomerate and do not form a stable suspension. Based upon the teaching of the above referenced German Patent No. DE 27 14858 C3 and similar references, it seem logical to assume that the application of high shear would result in the formation of stable suspensions. However, when high shear is applied to such unstable suspensions, the Ca(OH)2 particles do not form stable, non-settling suspensions as would be expected. When the particle size of such normal hydrated lime is reduced either by air classification, milling or other means, it still does not form stable, non-settling calcium hydroxide suspensions.